1. Field of the Invention
This invention relates to a smectic C liquid crystal compound and its composition. More particularly, it relates to a smectic C liquid crystal compound and its composition and a ferroelectric smectic C liquid crystal composition using the same and further a light-switching element using the composition.
2. Description of the Related Art
Liquid crystal compositions have been broadly used as a display element material. Most of the current liquid crystal display elements are those of TN type display mode, and this display mode utilizes nematic phase.
The TN type display mode used for the liquid crystal display is roughly classified into two modes. One mode is an active matrix mode having switching elements fixed to the respective pixels. An example of this mode is a mode using a thin film transistor (TFT). The display grade has reached a level matching that of CRT (cathode ray tube), but it is difficult to bring the picture surface into a large scale and the cost therefor is high.
Another mode is STN (super twisted) mode. The contrast and the visual sensation-dependency have been improved as compared with those of conventional simple matrix mode, but the display grade has not yet reached the level of CRT, but its cost is cheap. These two modes have a merit and a demerit when its grade and production cost are taken into consideration.
A mode expected ten years ago to solve the two problems is a mode of ferroelectric liquid crystal (FLC). At present, what is merely called FLC, refers to a surface-stabilized ferroelectric liquid crystal (SSFLC). This SSFLC was proposed by A. Clark and S. T. Lagerwall (see Applied Physics Letters, 36, 899, 1980). Since then, it has been called a liquid crystal of the next generation, and its commercialization has been tried by appliance makers and material makers, as well as improvement of the characteristics and commercialization have been made.
Because, ferroelectric liquid crystal elements are provided with the following characteristics in principle:
(1) high speed response properties,
(2) good memory properties, and
(3) broad viewing angle.
The above characteristics suggest a possibility of SSFLC into a large capacity display and have made SSFLC very attractive.
However, as the research has advanced, problems to be solved have been clarified.
Among the problems, a stabilized exhibition of memory is the first problem. As the causes of the difficulty in the stabilized exhibition of memory, non-uniformity of smectic layer structure (for example, twisted alignment, chevron structure) and occurrence of the inside electric field considered to originate from the excess spontaneous polarization, etc. have been considered.
As a means for exhibiting stabilized memory properties, a method of using a ferroelectric liquid crystal composition having a negative dielectric anisotropy has been proposed (see Paris Liquid Crystal Conference, p. 217 (1984)). This method has been referred to as AC stabilizing effect.
Liquid crystal molecules having a negative .DELTA..epsilon. value have such a property that when an electric field is impressed in a vertical direction to the electrode in a cell subjected to a homogeneous alignment treatment, the molecules are directed in a state parallel to the glass substrate (the parallel axis of the molecules is directed vertically to the direction of the electric field). When a low frequency electric field is impressed, the spontaneous polarization replies to the electric field; hence when the direction of the electric field is inverted, the liquid crystal molecules, too, follow the inversion and move to another stabilized state, where they become a state parallel to the substrate due to the effect of .DELTA..epsilon.. Whereas, when a high frequency electric field is impressed, the spontaneous polarization cannot follow the inversion, but only .DELTA..epsilon. is effected; hence even when the direction of the electric field is inverted, the liquid crystal molecules do not move and become parallel to the substrate, as they are. This is a mechanism of exhibiting the memory properties utilizing an AC stabilizing effect. A high contrast is thereby obtained. This concrete example has already been reported (see SID '85 digest, p.128, 1985).
Further, "a method of utilizing a liquid crystal material having a negative dielectric anisotropy" has been separately proposed by Surguy et al. (P. W. H. Surguy et al., Ferroelectrics, 122, 63, 1991). This technique is a promising one for realizing the high contrast, and P. W. Ross, Proc. SID, 217 (1992) discloses a ferroelectric liquid crystal display employing this technique. This ferroelectric liquid crystal display will be described below in more detail.
In the case of a conventional ferroelectric liquid crystal material whose dielectric anisotropy is not negative, as the voltage (V) becomes high, .tau. (a pulse width necessary for effecting memory) lowers monotonously. Whereas, in the case of a ferroelectric liquid crystal material having a negative anisotropy, .tau.-V characteristic showing a minimum value (.tau.-V min) is obtained. Surguy et al. have reported JOERS/Alvey driving method as a driving method utilizing this characteristic.
The principle of this driving method refers to a method wherein, when a voltage of .vertline.Vs-Vd.vertline. is impressed, a memory state of a ferroelectric liquid crystal element is switched, and when a voltage of .vertline.Vs+Vd.vertline. higher than the above voltage is impressed, and when a voltage of .vertline.Vd.vertline. lower than the above voltage is impressed, the memory state is not switched.
Since the ferroelectric liquid crystal material having a negative dielectric anisotropy can be applied to a display element utilizing the AC-stabilizing effect and the T min, as described above, the above material has potentially a possibility of being practically utilizable.
However, the response speed of the ferroelectric liquid crystal material used for the above element utilizing .tau. min is still low. Further, Vs+Vd, too, is as high as 57.5 V to 60 V; thus it has not yet reached a practical level. According to the report of Ross et al. (P. W. Ross, Proc. SID, 217 (1992)), the driving voltage of the ferroelectric liquid crystal display prepared for trial is 55 V. As to the cost of the IC driver for driving the ferroelectric liquid crystal display, the higher the voltage, the higher the cost. Thus, the high driving voltage becomes a serious cause of increased cost. In order to prepare a ferroelectric liquid crystal display having a relatively low cost, it is necessary to drive it using a general-purpose IC driver whose cost is not so high; thus it is necessary to suppress the driving voltage down to 40 V or lower. The reason for needing a high driving voltage at the present time is that the voltage value (V min) in .tau.-V characteristic is high; hence in order to drive the display at 40 V or lower, it is necessary to develop a ferroelectric liquid crystal material exhibiting a V min of 35 V or lower.
According to Surguy et al., V min is obtained by the following equation: EQU V min=E min.multidot.d=Ps.multidot.d/(.sqroot.3.multidot..epsilon..sub.o .multidot..DELTA..epsilon..multidot.sin.sup.2 .theta.).
In the above equation, E min refers to the minimum value of electric field intensity; d refers to cell thickness; Ps refers to spontaneous polarization value; .DELTA..epsilon. refers to dielectric anisotropy; and .theta. refers to tilt angle. As seen from this equation, in order to make the V min. lower voltage, a larger negative dielectric anisotropy and a less spontaneous polarization value are required. However, since the response speed of the ferroelectric liquid crystal is related to the spontaneous polarization value, if the spontaneous polarization value is reduced, it is difficult to obtain a high speed response. Accordingly, for the liquid crystal material, a low viscosity material having a negative dielectric anisotropy is required.
The present inventors have already filed a patent application directed to a ferroelectric liquid crystal composition suitable to drive utilizing the AC-stabilizing effect (Japanese patent application laid-open Nos. Hei 1-168792, Hei 1-306493 and Hei 4-4290). However, the response speed thereof has not yet been fully practical.
The object of the present invention is firstly to provide a smectic C liquid crystal composition having a negative .DELTA..epsilon. and making the high speed response possible, secondly to provide a ferroelectric chiral smectic C liquid crystal composition using the same, and thirdly to provide a liquid crystal display element using the above ferroelectric chiral smectic C liquid crystal composition and the driving method thereof.